Fabrication of advanced electronic and opto-electronic integrated circuits often involves the growth of a multi-layer structure of different compositions, for example III-V semiconductor compounds and their alloys. Sometimes the composition or thickness of the layers is critical and must be well controlled. For instance, quantum-well semiconductor lasers may use In.sub.x Ga.sub.1-x As in a very thin active layer to emit light. Both the composition or alloying ratio x and the layer thickness will determine the emission wavelength. The procedure of post-growth characterization of composition and growth rate to readjust the deposition parameters for future depositions, although the standard practice in the past (U.S. Pat. Nos. 4,483,725 to Chang and 4,786,616 to Awal et al.), is becoming increasingly unsatisfactory as the complexity of the structures increases (e.g., quaternary compositions) and the required accuracy of growth becomes more demanding. Even once the process parameters have been established, uncontrolled, and possibly uncontrollable, variations in the growth process may introduce enough variation to put a device out of specification, thus reducing the device yield.
Many measurement techniques are amenable to in situ characterization in which the sample is measured while in the growth chamber at ambient pressures comparable to those during the growth process. Thereby, surface contamination and oxidation are minimized. However, if the in situ measurement requires interruption of the growth, it neither accurately characterizes an uninterrupted growth nor is it satisfactory for production-line use.
Real-time characterization of layer deposition, that is, in situ measurement of the growing surface during continuing growth and without interruption, provides a better understanding of the growth process. However, using that characterization for real-time control of the growth has seldom been implemented. Aspnes et al have disclosed real-time growth control in U.S. patent application, Ser. No. 07/255,140, filed Oct. 7, 1988. They relied on reflectance difference spectroscopy, which measures the difference in reflected intensities for two polarizations of light incident upon the growing surface. Their technique effectively characterized the near surfaces of III-V semiconductors, specifically, whether the uppermost layer was a group-III cation layer or a group-V anion layer. Therefore, they suggested using their spectrometer to interrupt the supply of growth species in atomic layer epitaxy, in which a crystal is grown an atomic layer at a time. Reflection high-energy electron diffraction (RHEED) provides similar information about the near-surface region. Both techniques could in principle be used by integrating over time to obtain thickness and composition information of a growing layer. Atomic layer epitaxy is, however, considered too difficult and slow for production-line semiconductor fabrication.
Optical ellipsometry has long been recognized as providing accurate characterizations of semiconductors and other materials, their surfaces, layer compositions and thicknesses, and overlying oxides. Ellipsometry probes the surface to within an absorption length of the measuring radiation, a useful range for layer growth. As will be discussed later, it can measure the real and imaginary effective dielectric constants, which can be interpreted in terms of layer thickness and composition. The probing wavelength may be changed in spectro-ellipsometry (SE), thus varying the absorption length and the dependence of the data on the composition of the material. Hottier et al have disclosed real-time ellipsometric characterization in a technical article entitled "Surface analysis during vapour phase growth", appearing in Journal of Crystal Growth, volume 48, 1980 at pages 644-654. In a related article entitled "Qualitative and quantitative assessments of the growth of (aluminum, gallium) arsenide-gallium arsenide heterostructures by in situ ellipsometry" appearing in Revue Physique Applique, volume 16, 1981 at pages 579-589, Laurence et al emphasized that post-experiment calculations were required to derive the composition and thickness of grown layers.
Drevillon discussed in situ ellipsometry in a technical article entitled "In situ analysis of the growth of semiconductor materials by phase modulated ellipsometry from UV to IR" appearing in Surface and Interface Analysis of Microelectronic Materials Processing and Growth, SPIE Proceedings, volume 1186, 1989 at pages 110-121. He concluded the article with the observation that his work demonstrated the potential of ellipsometry for in situ process control.
Robillard has disclosed the use of ellipsometry for the control of thin film growth in U.S. Pat. No. 4,434,025. However, Robillard was concerned primarily with the amorphicity or crystallinity of the film, and thus with the growth temperature. He could thus derive sufficient control information from a single set of ellipsometric data taken for all analyzer angles. He also measured the film thickness by comparing relative intensities of two peaks in the set, which peaks had been equalized in a calibration step prior to thickness measurement. Robillard's technique however lacks control over the film composition.